A satellite communication system requires one or more spare or back-up satellites in orbit in order to provide a high reliability service with a minimum of down time. Conventionally one such spare or back-up satellite is located close to the operational satellite but at a different longitude. This has the advantage of preventing collision between the operational and spare satellites but if the spare or back-up satellite is required to take over service from the operational satellite several days can elapse before the spare satellite can be brought into station and made operational.